I looked at my test images from the S1R multishot mode (borrowed S1R in front of a local camera shop). The multishot mode definitely cannot be handheld, even under ideal conditions. I was using a 1/100 of a second shutter speed, f4, ISO 100 pointed at a brick wall about 10 feet away. It took close to 1 second to get all the images - it seems to shoot at the full 9 fps.
In the thumbnail, the result looked like what I would have expected from handholding at a full second without IS - about the expected amount of blur if the shutter had been open that long. Upon a closer examination, there was an additional artifact - a diagonal striping effect. If you think about what it's actually doing, the striping makes sense - it is adding several (individually sharp) exposures together, slightly off-register. The direction of my movement must have been on that diagonal. It also records one frame (presumably the first ) separately, which was sharp, as expected.
If the multi-shot mode cannot be handheld at all, that probably puts some limit on its ability to handle subject motion as well - whatever motion there is has to be much less than the effect of very steady handholding (I had braced up pretty darned well, because I was trying to see what happened with a minimal amount of motion).
Another interesting result was the sharpness of the Panasonic 50mm f1.4 lens. I also had my own Nikon Z7 out there, with the store's 50mm f1.8 Nikkor Z on it (at exactly the same settings - f4, 1/100, ISO 100), to control against my screwing up with the unfamiliar camera. If the non-multishot Panasonic shots had been lousy, but the Nikon shots had been sharp, I would have known the error was mine - that I had somehow mishandled the S1R. The little Nikkor was actually slightly sharper than the much larger and more expensive Panasonic lens in the center of the frame (both were very sharp, the Nikkor just had a bit more "bite"). Both were quite close to equal (and very good) at the frame edges. In the extreme corners (1% of the frame), the Panasonic had a clear advantage.
Other than the extreme corners, though, the compact, relatively inexpensive Nikkor was holding right in there with the Otus-sized Panasonic lens. Of course, this is far from a perfect test - there is only so much you can tell from shooting a brick wall, handheld from 10 feet away with one copy of each lens. Brick walls have no bokeh opportunities, I have no idea if I had a great Nikkor and a lousy Panasonic lens or the other way around, etc. It certainly bodes well for the Nikkor, though (and the Panasonic, but a 1 kg, $2400 50mm had better be a great lens).
